Question

OO Design question.

I want to inherit singleton functionality into different hierarchies of classes. That means they need to have their own instance of singleton per hierarchy.

Here is a brief example of what i am trying to do:

class CharacterBob : public CCSprite, public BatchNodeSingleton {
 ... 
}

class CharacterJim : public CCSprite, public BatchNodeSingleton {
 ...
}


class BatchNodeSingleton {
public:
    BatchNodeSingleton(void);
    ~BatchNodeSingleton(void);

    CCSpriteBatchNode* GetSingletonBatchNode();
    static void DestroySingleton();
    static void initBatchNodeSingleton(const char* asset);

protected:
    static CCSpriteBatchNode* m_singletonBatchNode;
    static bool singletonIsInitialized;
    static const char* assetName;
};

This code will cause Jim and Bob to share BatchNodeSingleton’s protected members. I need them to have their own set each. What would be a good solution? Collection of pointers that can be looked up by assetName as key?

Would really appreciate your thoughts.

Answer 1

CRTP is a popular pattern:

template <typename T> struct Singleton
{
    static T & get()
    {
        static T instance;
        return instance;
    }
    Singleton(Singleton const &) = delete;
    Singleton & operator=(Singleton const &) = delete;
protected:
    Singleton() { }
};

class Foo : public Singleton<Foo>
{
    Foo();
    friend class Singleton<Foo>;
public:
    /* ... */
};

Usage:

Foo::get().do_stuff();